In the treatment of tumors by ionizing radiation, x-rays or gamma rays are particularly used. The ideal in radiation therapy of malignant disease is achieved when the tumor is completely eradicated and the surrounding normal tissue, in the treated volume, shows little or no evidence of structural or functional injury. The important factor in successful treatment is the difference in radiosensitivity of neoplastic and normal cells. All tissues, normal and neoplastic, are affected by radiation so that radiosensitivity is a relative term. The basis of radiation therapy is that cells that are actively proliferating or that are of a primitive type are more sensitive than normal tissue so that there is usually a considerable margin between doses that are damaging to neoplastic and to normal cells. The difference depends on the capacity for intracellular repair of normal and neoplastic cells and the ability of normal organs to continue to function well if they are only segmentally damaged. If surrounding tissue can tolerate twice the radiation dose of a given tumor, then the tumor is radiosensitive.
Mammalian cells are capable of accumulating radiation damage before they are killed exponentially. Also, if allowed sufficient time after exposure, mammalian cells are capable repairing sublethal and potentially lethal radiation damage. The effects of x-rays of gamma rays on growing cells vary with intensity and duration of exposure and consist of destruction of some cells, inhibition of imminent mitosis; followed by abnormal mitosis and disruption of the cells and damage to resting cells so that continued proliferation fails. The prime target of present radiotherapy is the DNA molecule of a cell which does not select for cancer cells but selects for DNA repair capabilities. Even a two-to-one increase in radiation sensitivity in cancer cells will result in a curable condition. However, normal surrounding tissue may not be more tolerant to x-ray therapy than cancer tissue which makes this therapeutic modality useless.